If you can answer "yes" to any of the following questions, your computer might have a virus.
Is your computer running very slowly? A common symptom of a virus is much slower than normal computer performance. However, there can be other reasons for slow performance, including a hard disk that needs defragmenting, a computer that needs more memory (RAM), or the existence of spyware or adware. For more information about spyware, see How to tell if your computer is infected with spyware.
Are you getting unexpected messages, or are programs starting automatically? Some viruses can cause damage to Windows or some of your programs. The results of this damage might include messages appearing unexpectedly, programs starting or closing automatically, or Windows shutting down suddenly.
Is your modem or hard disk working overtime? An e‑mail virus works by sending many copies of itself by e‑mail. One indicator of this is that the activity light on your broadband or external modem is constantly lit; another is the sound of your computer's hard disk continually working. These are not always symptoms of a computer virus, but when combined with other problems, can indicate a virus infection.
To check for viruses, scan your computer with an antivirus program. New viruses appear every day, so keeping your antivirus program updated is important. For more information about computer security, go to the Security at Home page on the Microsoft website. To learn how to remove malicious software from your computer, go to the Microsoft Malicious Software Removal Tool webpage.